The seat has been relative to others a marginal seat since 2010 as its winner's majority has not exceeded 4.6% of the vote since the 17.3% majority won in that year. The seat has changed hands once since that year.

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1885-1918: In the Borough of Belfast, that part of Cromac ward not in the constituency of Belfast East, that part of St. George's ward not in the constituency of Belfast West, and the townlands of Malone Lower and that part of Malone Upper within the parliamentary borough in the parish of Shankill.

1922-1974: The County Borough of Belfast wards of Cromac, Ormeau, and Windsor.

1974-1983: The County Borough of Belfast wards of Cromac, Ormeau, and Windsor, the Rural District of Lisburn electoral divisions of Ardmore, Dunmurry, Finaghy, and Upper Malone, and the Rural District of Hillsborough electoral divisions of Breda and Edenderry.

Prior to the 2010 general election the Boundary Commission proposed expanding Belfast South further into Castlereagh, taking in areas currently contained in both Strangford and Belfast East. This was strongly opposed by the DUP but supported by the Ulster Unionists. It was also one of the issues which generated the most negative comments in the written submissions with a petition representing half of Cregagh's residents opposing its move.

Following the local enquiries, the Boundary Commission proposed retaining the Cregagh ward in East Belfast while transferring instead the Hillfoot ward. These proposals were submitted as final recommendations and were put into force through the passing of the Northern Ireland Parliamentary Constituencies Order[1] in 2008. The electorate of the seat falls below the electoral quota for all seats in Northern Ireland, itself less than the average for all seats across the UK, as such its making by international standards was very slightly malapportioned.

Belfast South, centred on the River Lagan contains some of Belfast's most exclusive residential districts as well as Queen's University Belfast, and the overall tenor of the constituency is middle-class – young, trendy and cosmopolitan towards the city centre, with Northern Ireland's biggest concentrations of both students and ethnic minorities, and settled and prosperous further out. Despite this, significant pockets of inner-city working class areas such as the Markets and a number of isolated suburban estates are in the constituency.

There has been particularly rapid demographic change in Belfast South over the past 20 years, change which seems to be continuing. Since the 2011 census, Belfast South consists of a slightly larger Catholic population than Protestant. Belfast South traditionally had a unionist majority, but the nationalist vote has now surpassed this in more recent elections. There have also been strong votes for other parties such as the Alliance Party of Northern Ireland, Green Party, the Conservatives and the Northern Ireland Women's Coalition. The seat has also seen a steady series of candidates backed by groups who aspire to support the British Labour Party despite its prior ban on membership and organisation in Northern Ireland, though their results have been minimal. Until the 1990s the main focus of attention has been on contests between unionist candidates.

In January 2005 Smyth announced that he would be retiring at the 2005 general election, raising speculation both as to whom the Ulster Unionists would field in succession to him and what effect a different candidate would have upon their share of the vote. The UUP selected Assembly member Michael McGimpsey, albeit with a highly controversial and bitter selection. The aftermath saw McGimpsey repudiated by many prominent local and national Ulster Unionists, including both Smyth and former UUP leader James Molyneaux. The DUP selected Jimmy Spratt and offered an electoral pact to the UUP that would give each party a free run at one out of South Belfast and Fermanagh and South Tyrone. This offer was rejected by the UUP.

In the event, the DUP and UUP both fielded candidates which split the vote, while the nationalist vote mainly went for the SDLP over Sinn Féin, with the result that the SDLP took the seat despite a majority of votes cast for unionist candidates.

In 2010, Sinn Féin opted not to stand against the SDLP to avoid splitting the nationalist vote. This effort was successful as the SDLP won the seat with a majority of 6,000. This was the seat in which the Alliance Party had their second-best showing, polling 15% of the votes. Alasdair McDonnell retained the seat in May 2015, with only 24.5% of the vote. This is the smallest proportion of the vote a winning candidate has ever achieved in a UK general election.[2]

In 2017 the seat was won by Emma Little-Pengelly of the DUP[3] with Alasdair McDonnell losing his seat along with all other SDLP MPs in Northern Ireland.[4]